It's a good question. One of Apple's more formidable strengths is its vast audience of longtime iPhone users. People tend to buy a new phone every two or three years, so the historic expectation has been that the installed base will drive future sales.

Some analysts even expected a "super-cycle" to emerge last year as older iPhone users re-upped into the new iPhone X.

Instead, sales went down.

The installed base isn't coming to the rescue anytime soon, either. "Despite this strong growth in installed base, we ... do not expect iPhones to grow in FY 18, the third straight year of flat-to-declining units," Sacconaghi wrote in a recent note to clients.

Now, Sacconaghi believes he has figured out why it is that the "installed base" can grow at 30% while new unit sales are sagging. The base is being fuelled by used, second-hand and resold devices. New iPhone replacement sales are essentially flat, and all the growth is in the second-hand market, Sacconaghi says: